All DSSAT models share the soil water balance subroutine. On a daily basis, the soil water balance is computed by adding irrigation and rainfall and subtracting surface runoff, drainage, plant transpiration, and soil evaporation. Within a soil column, soil water is redistributed by vertical drainage, capillary rise, and tillage. Rainfall is supplied as a user input in weather files. Irrigation is input via the experimental details file and includes the type of irrigation, the efficiency of water supply, and the date and amount of irrigation applied. Rainfall is partitioned to infiltration and surface runoff via the SCS curve number approach (Ritchie, 1998).

Drainage of soil water uses a tipping bucket approach for layered soils and assumes only one-dimensional flow (Ritchie, 1998). Successive soil layers are defined by the lower limit (wilting point), drained upper limit, and saturated volumetric soil water content. Downward water movement depends on a soil drainage factor (fraction per day), limited by the saturated hydraulic conductivity of the layers. Actual evapotranspiration (ET) depends on total ETo demand using either of two user-selectable options: Priestley-Taylor (1972), based on standard weather data input, or FAO-56 (Allen et al., 1998), which requires wind speed and relative humidity as input data. After it is calculated, ETo is partitioned to the potential transpiration of the crop canopy (Ep) or potential evaporation of the soil (Es) as a function of the LAI and an energy extinction coefficient (Kep). Kep differs for each crop in CROPGRO, but it is more complex for the CERES crops as a “mixed” function of extinction of photosynthetically active radiation is used. Actual soil evaporation depends on the potential Es and the soil water content, using either the older Stage 1 (square root of time method) or the Suleiman-Ritchie method (Ritchie et al., 2009). Actual transpiration of the crop is the minimum of the potential Ep or the water uptake. Potential root water uptake from successive layers follows Ritchie (1998) and depends on the root length density and the fraction of available soil water content in each layer. Total root water uptake is then integrated over all layers, and transpiration is reduced if potential root water uptake is less than potential Ep. The daily photo-assimilation is reduced as a function of actual transpiration (root uptake) over potential Ep, using a drought stress factor called SWFAC. Expansive processes are reduced somewhat sooner by a similar factor called TURFAC. See Boote et al. (2009) for a review of water balance, evapotranspiration, and simulation of water stress effects in the CROPGRO model.